Monday, April 30, 2012

More unsettled weather Tuesday.. thunderstorms possible..

A low pressure system over southern Saskatchewan will push a warm front across southern MB tonight, bringing another round of scattered showers into the Red River valley. There's even a chance of some scattered thunderstorms overnight as more humid air spreads in. Showers and scattered thunderstorms will taper off Tuesday morning with cloudy to partly sunny skies expected through midday. Additional showers and thunderstorms are possible later in the day into the evening, especially southeast of Winnipeg. In fact, some strong to possibly severe thunderstorms are forecast for areas south of the international border Tuesday, including much of Minnesota.

That elevated convection is associated with moisture convergence on the back edge of the LLJ, and the low at 925 to 850 hPa. Similar to an event we had back in March. Most aggresive moisture transport is off into Minnesota and NW Ontario, where better organized precipitation is occurring.

A few cells have gone up south and east of Brandon. Looks like there's between 1,000 and 1,500 J/KG of SBCAPE out there. Weaker instability exists to the east. We'll have to see if instability can increase with eastward extent and if the convection survives..

Murky, complex pressure pattern at the surface.. with a warm front along the TransCanada and trough connecting at least two surface lows over SW Manitoba, and NE S Dakota. Moisture convergnece along the back edge of the LLJ and the northern low, combined with CAPE of 100-1500 J/kg triggering area of conbvection.

Cells are advecting towards Winnipeg, but encountering more stable air.. CAPE really drops off north of the warm front, where it was cloudy most of the day. Uncertainty regarding how far north instability (750-1000 J/kg)can be advected, and if additional cells will form along the warm front. At the very least some showers will be around the region..

What looked like a solid area of steady rain, parted right over Winnipeg as we have seen several times before.

Area of stratiform precip advecting NE with the mean flow. Meanwhile line of thunderstorms from Steinbach thru Emerson, moving more to the east as new cells build back and propagate down into the inflow of moist, unstable air.

Something strange about the reflectivities on Foxwarren radar.. they look way overdone. Line of storms went through Brandon with a little heavy rain and some gusty winds (23 knots) but nothing major. Storms nowhere near as intense as they move into Woodlands radar range. Foxwarren radar was having some issues past few days.. I wonder if it's not calibrated properly.

I'm still questioning the radar returns out of Foxwarren.. they seem to be giving reflectivity values higher than neighbouring radars. Couple of cells on southern flank may be producing marginally severe hail, but it looks like Foxwarren is overdoing the radar returns.

Arctic front along the TransCanada, with afternoon temps of only 5 C at Dauphin. Meanwhile, tropical warm front is from southern Minnesota back to extreme southern N Dakota.

Northern stream wave back over Montana generating some scattered convection over central N Dakota. Models like NAM show an area of precipitation associated with the leading edge of that disturbance moving off to our SE tonite.

However best forcing and LLJ will be well off into the central plains and midwest.. as energy consolidates around a more focused upper wave over the central plains.

American models (NAM and GFS) show no precipitation for us tomorrow, as high pressure and arctic front slump south..

Tonight's RGEM has really cut down the rain amounts over southern MB for Thursday.. now only showing a couple of mm at most. RGEM was the most aggressive model up to tonight giving 10-20 mm across southern MB. Looks like it's trended to what American and ensemble guidance were suggesting.. that is, not much precip Thursday. Still cloudy and on the cool side, before a better chance of showers on the weekend..

Looks generally cool and unsettled next few days. Weak impulses in SW upper flow will bring scattered but generally light shower activity over southern MB, especially Friday into Friday night, with heavier convective activity possible over the Dakotas. Main upper trof moves through Sunday into Monday when we'll have a better chance of more widespread rain over us, possibly enhanced with some thunderstorms. Heaviest rainfall over next 5 days looks like it will be concentrated mainly to our south in the Dakotas, and west through srn SK. Hopefully we get into a warmer drier pattern by the middle of next week.

As cloudy and showery as its been, precipitation amounts for areas around Winnipeg have been paltry so far.. in the 5-7 mm range. As soon as we get sun and drier RHs, topsoil will rapidly dry out again.

As Rob mentioned, precipitation will once again be focused to our south and west. Longwave pattern re-amplifies with upper ridge buidling into NW Ontario and NE Manitoba, as parent circulation becomes closed off and vertically stacked.

At the same time, shortwaves will undercut the upper-ridge to our south, tapping good moisture and instability. First wave comes thru overnite into tomorrow, and second on Saturday nite. Looks as though first wave will not able to overcome dry NE flow from a surface high over Hudson Bay (you guessed it).

NAM and GFS show more robust area of precipitation for second system.. with an area of precipitation between the northern plains wave and the main upper low. Typically in these situations, precipitation splits so QPF might be overdone for our region.

Uncertainty regarding evolution of the parent low. GFS weakens blocking pattern and opens up low to a wave.. area of wrap-around moisture may slump over our region by late in the weekend and early next week.

Longwave pattern has been flipping from a moist SW flow, to a droughty, coolish omega block for us... without either one becoming dominant so far. I would not put too much stock into longer range model solutions given this variablity.

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